1. Technical Field
The present invention relates to image processing for binarization using a clustered dither. Specifically, the invention relates to image processing capable of improving granularity of an image formed on a print media.
2. Related Art
An image forming device having ink ejecting mechanism, such as an ink jet printer, has a plurality of nozzles for ejecting an ink dot in a droplet form, in order to perform high-speed printing. The nozzles are mounted on a printing head. An ink jet printer causes the printing heads to eject ink dots by moving a sheet of paper (or a print media) in a sheet feeding direction and moving the printing heads repeatedly in a direction orthogonal to the sheet feeding direction, thereby forms images on the paper. This kind of printer is referred to as a “multi-pass printer”.
The printing speed of a multi-pass printer is restricted since the multi-pass printer needs a two-directional scan. Therefore, an improvement in printing speed is required.
Reducing the scan dimension into a one-dimensional scan, for example, scanning only in a sheet feeding direction, referred to as single-pass printing, is one of the approaches to improve printing speed. It is necessary for single-pass printing to extend the size of a printing head greater than the width of the printing paper. This kind of printer, in other words, a printer having a printing head which is larger than the width of printing paper, is referred to as a “line head printer”. In a printing head of a line head printer, it is necessary to layout nozzles at a regular distance from each other on the printing head. However, it is difficult to fabricate a printing head in which the nozzles are laid out at exact intervals, because of fabrication errors.
The dispersion of a distance between two adjacent nozzles causes the actual position of an ink dot formed on printing paper to be different from an ideal position. Furthermore, a skew of a nozzle also causes the actual position of an ink dot formed on printing paper to be different from an ideal position. The phenomenon of causing a difference between the ideal position and the actual position, is called “splash bending”. In a single-pass printer such as a line head printer, the phenomenon of splash bending causes white bands or dark bands to appear on the print product. The white band appears where a distance between two adjacent nozzles is greater than the ideal distance, and the dark band appears where a distance between two adjacent nozzles is less than the ideal distance. This phenomenon is referred to as “banding”.
To prevent deterioration in image quality, some technologies have been developed. For example, JP-A-9-107473 discloses a “clustered dither” as a binarization algorithm. Furthermore, JP-A-2001-177722 discloses an image processing that is a combination of an error diffusion method and a dither method. According to JP-A-2001-177722, concentrated dots are formed in low and medium density, and dispersed dots are formed around the concentrated dots in high density.
Some ink jet printers can control a dot radius of an ink dot ejected by a nozzle, in other words, a dot size of an ink dot formed on the printing paper. In an ink jet printer capable of ejecting an ink dot whose size is either one of the sizes S, M, and L, the image quality is dependent on an algorithm for generating an ink dot having one of the three sizes, in other words, an algorithm for determining where a dot is to be formed and which size of dot is to be formed. JP-A-2001-177722 has a problem in that the granularity of the image deteriorates because the concentrated dots are generated in low and medium density. A further drawback in JP-A-2001-177722 is that, it discloses a method of image processing performed only with single-size ink dots, and not a multi-size ink dot.